[Fun with c++] Adapter model for reverse iterators

Topic of this issue: Implementation of reverse iterator in STL

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1. Reverse iterator

Reverse iterator, let's think about it. At the beginning, we implemented the forward iterator by using class templates or directly using native pointers. Copy
a copy of the iterator we wrote before, and then ++ becomes -- , - - Change to ++.

We will find that when this is implemented, each container has to write such a thing. The code is very redundant.
So we implement a general reverse iterator, that is, if there is a forward iterator, directly adapt to the Our reverse iterator.

Is there any other way?
We can use the adapter directly to adapt the forward iterator to a reverse iterator. That's it.

2. Source code

With the big idea, get started directly.
The implementation process is analyzed.

//反向迭代器的适配器 。


namespace zxf
{
    //调用的时候，一般都是在别的类内部使用，供别的类调用
    //typedef reverse_iterator<iterator> reverse_iterator;
    //typedef reverse_iterator<const_iterator> const_reverse_iterator;
    //这里就是使用 iterator 适配 reverse_ierator
    //使用const_iterator 适配 const_reverse_iterator

     template<class iterator, class ref, class ptr>
    class reverse_iterator
    {
    public:
        typedef reverse_iterator<iterator, ref, ptr> self;
        //迭代器在定义的时候 必须初始化。
        reverse_iterator(const iterator it)
            :_it(it)//注意只能写在初始化列表中，不能写在函数体内部
            //_it是一个正向迭代器，迭代器在创建的时候必须初始化。
        {}
        self operator++(){
            --_it;
            return *this;
        }
        self operator--(){
            ++_it;
            return *this;
        }
        
        //iterator也是一个类模板，它是对 pnode（node*）（list就是节点的指针，vector就是元素的指针） 的封装
        //iterator知道数据类型，但是reverse_iterator不知道数据类型。

        ref operator*(){//这就是为啥要给出第二个模板参数的原因之一：
            //我们无法知道iterator类模板成员变量指向的元素的类型，所以要把那个元素的类型传过来。
            //另一个原因就是区分开我们的const反向迭代器和正常反向迭代器。
            iterator tmp = _it;
            return *(--tmp);
            //这里'--'的原因就是我们在设置反向迭代器的rbegin() 和 rend()是怎么实现的
            //这就是一个边界的问题，和 rbegin() ,rend()  结合看的。
            //假如你改变 自己实现的 rbegin() ,rend() 也可以不 '--'。
        }
        bool operator!=(const self& it)//这里引用可加可不加
        {
            return _it != (it._it);
        }

    private:
        iterator _it;//底层的正向迭代器。
    };
}

3. What is the meaning of the reverse iterator?

The key point is reuse. The reverse iterator here is not written for a certain container, but for all containers.
As long as a container has a forward iterator, it can be adapted to a reverse iterator.
Code repetition The usability is very good, and it is a very essential part of STL.